When the Arctic experiences summer, it is winter in Antarctica. So when sea ice reaches its annual low in the north, every September, it is at its maximum extent in the south. Different processes determine the extent of the ice in different seasons. In summer, melting is the driving factor. But the extent of winter ice is determined mainly by winds, which can either push ice out over a larger area or compress it into a smaller one.

Long-term trends, extrapolated from satellite observations that began in the late 1970s, show that the September ice extent has grown around Antarctica and shrunk in the Arctic.

So why are these trends going in opposite directions?

What's happening in the Arctic?

What about the Antarctic?

Is anything happening with the jet stream in the northern hemisphere?

Given that winter sea ice in Antarctica is growing, is the alarm about the Arctic overblown?

Can we at least be fairly relaxed about the changes in the southern hemisphere?

That would be unwise. The main concern is the long-term future of the much larger volume of ice on the continent of Antarctica itself, and in the ice shelves that form where its glaciers meet the sea – especially when the ozone hole heals and its cooling influence is diminished. The evidence points to continuing loss of Antarctic ice, which will lead to continuing sea level rise, says Sharon Stammerjohn of the University of Colorado at Boulder.